1. Field of the invention
The present invention relates to a transmission output detecting circuit used in a radio transmitter-receiver of a TDD (time-division duplexing) communication system, and particularly relates to such a transmission output detecting circuit in which loss of transmission power is eliminated when transmission output is detected.
2. Description of the related art
In mobile stations and base stations for mobile communication adopting a TDD communication system, the level of transmission power is changed adaptively so as to keep reception in the respective stations in the optimum level. For such a purpose, each of radio transmitter-receivers of these stations has a circuit for detecting its transmission output.
As shown in FIG. 4, a conventional radio transmitter-receiver of a TDD communication system having a transmission output detecting circuit has an antenna 1, an antenna filter 2 for restricting a signal band, a TDD switch 3 for switching connection between transmission and reception, a reception system constituted by a reception first-stage amplifier (reception top amplifier) 7 for amplifying a reception signal, a reception first mixer 8 for mixing an oscillation signal of a local oscillator 12 with the reception signal so as to convert the reception signal into a first intermediate frequency signal, an amplifier 9 for amplifying the first intermediate frequency reception signal, a reception second mixer 10 for converting the output of the amplifier 9 into a second intermediate frequency signal, and a demodulator 11 for demodulating the second intermediate frequency reception signal to a base band signal, a transmission system constituted by a modulator 6 for modulating a transmission signal, a transmission mixer 5 for mixing the oscillation signal of the local oscillator 12 with the modulated transmission signal so as to convert the transmission signal into a radio frequency signal, and a transmission power amplifier 4 for amplifying transmission power, and a transmission output detecting circuit belonging to this transmission system and constituted by a directional coupler 13 for making a portion of the transmission signal branch out, and a detector 14 for diode-detecting the branching transmission signal.
The TDD switch 3 of this radio transmitter-receiver is a high-frequency switch which is constituted by an FET, an PIN diode or the like, and performs switching between transmission and reception at a high speed in response to a control signal applied thereto. In a receiving period, a signal received through the antenna 1 is inputted to the reception top amplifier 7 through the antenna filter 2 and the TDD switch 3, and demodulated to a base band signal by the system constituted by the reception first mixer 8, the amplifier 9, the second mixer 10, and the demodulator 11.
On the other hand, in a transmitting period, a transmission signal modulated by the modulator 6 is converted into a radio frequency signal by the transmission mixer 5, amplified by the transmission power amplifier 4, and then transmitted from the antenna 1 through the TDD switch 3 and the antenna filter 2.
At that time, a part of the output of the transmission power amplifier 4 is branched out by the directional coupler 13, and performed diode-detection by the detector 14, so that the transmission output is detected. The level of the transmission power is adjusted by this detection output.
Instead of providing the directional coupler, a part of the transmission output may be simply extracted by capacity coupling.
In a conventional transmission output detecting circuit, however, a part of transmission output is extracted through a directional coupler or a capacity coupling connected to the output of a transmission power amplifier, so that a loss is caused in the transmission power and it becomes necessary to amplify the transmission power correspondingly.
Specifically, the detector 14 requires a power of about 10 dBm at maximum in accordance with the relationship between the detection sensitivity and the dynamic range of the detection circuit. Accordingly, measures are taken such that allowing for such a loss, the output is increased correspondingly in the power amplifier 4, or in the case of a comparatively middle or small power radio station the transmission power of which is 20 dBm or less, the transmission output extracted by the directional coupler 13 is amplified by an amplifier 15 before the transmission output is detected by the detector 14 as shown in FIG. 5, since the insertion loss is increased by the increase of the coupling factor of the directional coupler.
In either case, the conventional transmission output detecting circuit has a problem that its current consumption is increased at the time of transmission, and its circuit configuration is complicated.